1. Field of the Invention
The present invention relates to a ceramic metal halide lamp.
2. Description of Related Art
As a high-intensity discharge lamp (HID lamp), there exist, for example, a high-pressure mercury lamp, a high-pressure sodium lamp, a metal halide lamp and a ceramic metal halide lamp. The HID lamp is able to produce light by effectively utilizing discharges occurred between electrodes. Therefore, as compared with incandescent electric lamps, the HID lamp has various characteristics such as being large in luminous flux so as to become suitable for use in illumination of a large-scale area and being excellent in energy efficiency.
As to the HID lamp, in 1960s, there have been developed metal halide lamps using metal halide in which color rendering properties and luminous efficiency could be improved as compared with fluorescent mercury lamps.
In the metal halide lamp, since a luminous tube (arc tube) is made of a ceramic material and such ceramic material rarely reacts with the materials sealed into the inside of the luminous tube as compared with a prior-art luminous tube made of quartz, such metal halide lamp is able to use a wide variety of luminous materials. In addition, since the luminous tube is rarely deteriorated, it became possible to extend the life of a metal halide lamp. As a result, color rendering properties of the ceramic metal halide lamp could be improved and improvement of quality of light such as to stabilize a light color and an optical output could be realized and therefore the ceramic metal halide lamps were rapidly permeated into the market so as to illuminate the stores which need high color rendering properties.
However, in the ceramic metal halide lamp, lamp efficiency and color rendering properties are placed in a trade-off relationship and it has been said that it is difficult to improve both of the lamp efficiency and the color rendering properties at the same time. Accordingly, although prior-art ceramic metal halide lamps are famous for high color rendering properties and high lamp efficiency, they are inevitably classified into either ceramic metal halide lamps of the type that emphasizes on the lamp efficiency or ceramic metal halide lamps of the type that emphasizes on the color rendering properties. In general, “lamp efficiency” is expressed by lm/W (lumen per watt: brightness per watt) and “color rendering property” (the property of light source that affects color appearance of objects) is expressed by a general color rendering index (index indicative of color appearance) Ra. In this case, it has been customary that if the lamp efficiency is set to approximately η≧100, then a ceramic metal halide lamp is evaluated as being of the type which emphasizes on the lamp efficiency and that if the general color rendering index is set to approximately Ra≧80, then a ceramic metal halide lamp is evaluated as being of the type which emphasizes on the color rendering property.
It should be noted that the inventor of the present invention is not aware of the existence of a patent literature and a non-patent literature which may disclose a technology that can realize a ceramic metal halide lamp with high lamp efficiency and high color rendering properties by additionally sealing silver iodide (AgI) into a luminous tube which will be disclosed in the document of the present application.